fix(group): harden peak-rate config handling and label peak windows with server timezone

Follow-up fixes to #3569 based on code audit:

- Expose server_timezone / server_utc_offset in public settings (and the
  __APP_CONFIG__ injection payload) and label every peak-window display
  with the server UTC offset, so users don't misread the billing window
  as browser-local time
- Unify CreateGroup/UpdateGroup peak-config sanitization via a single
  NormalizePeakRateConfig chokepoint: non-subscription groups always get
  peak fields cleared; unparseable window strings and negative
  multipliers are scrubbed when peak is disabled
- Replace hot-path time.Parse in PeakMultiplierAt with a manual HH:MM
  parser (accept set verified byte-for-byte identical to
  time.Parse("15:04") by exhaustive fuzzing) and reuse it in validation
- Revert the zero-behavior CalculateCost indirection churn in
  billing_service/gateway_service introduced by #3569
- Remove dead GetGroupPlatformMap and the duplicate deref helper in the
  admin handler package
- Share frontend peak formatting via utils/peak-rate.ts, unify the ×N
  label format, and move hardcoded Chinese tooltips to i18n keys

frontend/src/components/common/GroupBadge.vue

@@ -30,6 +30,8 @@
 import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import type { SubscriptionType, GroupPlatform } from '@/types'
+import { useAppStore } from '@/stores/app'
+import { formatPeakRateWindow, serverTimezoneLabel } from '@/utils/peak-rate'
 import PlatformIcon from './PlatformIcon.vue'
 
 interface Props {
@@ -75,16 +77,26 @@ const hasCustomRate = computed(() => {
   )
 })
 
+const appStore = useAppStore()
+
 const hasPeakRate = computed(() => {
   return Boolean(props.showRate && props.peakRateEnabled && props.peakStart && props.peakEnd)
 })
 
 const peakRateText = computed(() => {
-  return `${props.peakStart}-${props.peakEnd} ×${props.peakRateMultiplier ?? 1}`
+  return formatPeakRateWindow(
+    {
+      peak_rate_enabled: props.peakRateEnabled,
+      peak_start: props.peakStart,
+      peak_end: props.peakEnd,
+      peak_rate_multiplier: props.peakRateMultiplier
+    },
+    serverTimezoneLabel(appStore.cachedPublicSettings?.server_utc_offset)
+  )
 })
 
 const peakRateTitle = computed(() => {
-  return `高峰倍率：${peakRateText.value}`
+  return t('common.peakRateTooltip', { window: peakRateText.value })
 })
 
 // 是否显示右侧标签

frontend/src/components/common/GroupOptionItem.vue

@@ -63,6 +63,8 @@ import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import GroupBadge from './GroupBadge.vue'
 import type { SubscriptionType, GroupPlatform } from '@/types'
+import { useAppStore } from '@/stores/app'
+import { formatPeakRateWindow, serverTimezoneLabel } from '@/utils/peak-rate'
 
 const { t } = useI18n()
 
@@ -99,16 +101,26 @@ const hasCustomRate = computed(() => {
   )
 })
 
+const appStore = useAppStore()
+
 const hasPeakRate = computed(() => {
   return Boolean(props.peakRateEnabled && props.peakStart && props.peakEnd)
 })
 
 const peakRateText = computed(() => {
-  return `${props.peakStart}-${props.peakEnd} ×${props.peakRateMultiplier ?? 1}`
+  return formatPeakRateWindow(
+    {
+      peak_rate_enabled: props.peakRateEnabled,
+      peak_start: props.peakStart,
+      peak_end: props.peakEnd,
+      peak_rate_multiplier: props.peakRateMultiplier
+    },
+    serverTimezoneLabel(appStore.cachedPublicSettings?.server_utc_offset)
+  )
 })
 
 const peakRateTitle = computed(() => {
-  return `高峰倍率：${peakRateText.value}`
+  return t('common.peakRateTooltip', { window: peakRateText.value })
 })
 
 // Rate pill color matches platform badge color